The present invention relates to a device for modifying and uniforming the distribution of intensity of a power laser beam.
It is known that the non uniform power distribution of the intensity of a laser beam represents a serious drawback for the possible industrial applications on the field of the thermal treatments. In fact, considering, for example, a process of hardening of steels or cast iron, it is noticed that an essential requirement consist in reaching a surface temperature value of the metal very near to the value of the melting temperature; in particular, the regions subjected to a laser treatment are required to have a uniform distribution of the temperature, because even small localized variations can determine an undesired hardness value or even an undesired fusion of the material. As these temperature variations are due to lack of uniformity in the spatial distribution of the intensity of the laser beam which acts onto the material, it is extremely important to have at one's disposal a laser beam of uniform intensity. Since it is not possible to act onto the source which emits the power laser beam, various devices have been provided which aim to render uniform the distribution of the intensity of the laser beam before said beam falls onto the material which has to be submitted to the treatment.
A first device, which is somewhat simple in its structure, comprises substantailly a converging lens, and the piece to be treated is positioned out of focus or in the focus of the lens; in this latter case, the lens utilized is of the type with a long focus. The device of this type allows an economical manipulation of the laser beam, however the obtained distribution of the intensity is still dependent on the shape and stability of the laser beam coming out from the emission source and therefore is highly critical as regards its practical utilization.
A second device, quite largely used, comprises substantially two oscillating mirrors which are mounted along the route of the laser beam and generally are distributed within the optical focusing device. In particular, these mirrors are made to oscillate in directions orthogonal to each other and allow obtaining an output beam whose cross-section size is adjustable. These devices allow obtaining also an improvement of the uniformity of the intensity profile of the outgoing laser beam in respect of the beam generated by the source, however this profile still has two undesirable peaks at its ends.
A third known device substantially comprises a curved integrator mirror which substantially divides the wave front of the incident laser beam into a plurality of secondary reflected beams and is dimensioned so as to direct all these secondary beams onto a single surface region of the piece to be treated. Said integrator mirror is formed substantially by providing a plurality of facetings on a copper mirror, for example by means of a diamond tool, or by fastening on a copper substratum a plurality of small plane square reflecting pieces, for example, of molybdenum. The mirror obtained by means of a diamond tool is substantially less expensive, but the intensity profile of the laser beam which is obtained from this mirror still has marked peaks uniformly distributed, because of the phenomena of interference and diffraction due to the typical surface configuration of the mirror itself. The mirror formed by means of small reflectant square pieces of molybdenum, in addition to being very expensive, has disadvantages which show themselves every time it is necessary to clean the square pieces because of the dust which easily penetrates into the slits between the small square pieces; moreover, it is particularly difficult to provide an efficient cooling system for this mirror.